The name change from Tharyx marioni to Aphelochaeta marioni occurred recently and some authors still use the previous name. Therefore, care should be taken when searching the literature on this species. In this review, where the species was researched under the former name, the species name is given as Aphelochaeta marioni (studied as Tharyx marioni). Aphelochaeta marioni is very difficult to identify (Mike Kendall, pers. comm.) and some authors (e.g. Farke, 1979) have commented that specimens that have been the subject of published research may have been misidentified.

Abundance
Gibbs (1969) studied the abundance of Aphelochaeta marioni (studied as Tharyx marioni) in Stonehouse Pool, Plymouth Sound. In silt/clay sediments at 5 m depth, the species occurred at a maximum density of 108,000 individuals/m2. In silt/clay and fine sand at the low water mark, the maximum density was 61,150 individuals/m2. Farke (1979) studied the abundance of Aphelochaeta marioni (studied as Tharyx marioni) in the Wadden Sea, Netherlands. In the intertidal, the maximum recorded abundance was 71,200 individuals/m2 in muddy sand.

FeedingAphelochaeta marioni is a deposit feeder, feeding at the surface of the sediment at night. While feeding the animal remains in its burrow and the two palps roam at the surface transporting sand, debris and diatoms to the mouth along a tentacle canal crenulated with cilia. Farke (1979) is unsure whether Aphelochaeta marioni is a selective feeder, but it seems not, as sand grains have been found in the gut of the animal.

Aphelochaeta marioni has been recorded from a variety of different sediment types. In the intertidal area of the Wadden Sea, it achieved highest abundance where the sediment fraction smaller than 0.04 mm diameter was greater than 10% of the total sediment (Farke, 1979). In the Severn Estuary, Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated, poorly sorted mud with relatively high interstitial salinity (Broom et al., 1991). In fact, Aphelochaeta marioni displays a remarkable tolerance for salinity range. Wolff (1973) recorded Aphelochaeta marioni (studied as Tharyx marioni) from brackish inland waters in the Netherlands with a salinity of 16 psu, but not in areas permanently exposed to lower salinities. Farke (1979) reported that the species also penetrated into areas exposed to salinities of 4 psu during short periods at low tide when the freshwater discharge from rivers was high.

The lifecycle of Aphelochaeta marioni varies according to environmental conditions. In Stonehouse Pool, Plymouth Sound, Aphelochaeta marioni (studied as Tharyx marioni) spawned in October and November (Gibbs, 1971) whereas in the Wadden Sea, Netherlands, spawning occurred from May to July (Farke, 1979). Spawning, which occurs at night, was observed in a microsystem in the laboratory by Farke (1979). The female rose up into the water column with the tail end remaining in the burrow. The eggs were shed within a few seconds and sank to form puddles on the sediment. The female then returned to the burrow and resumed feeding within half an hour. Fertilization was not observed, probably because the male does not leave the burrow. The embryos developed lecithotrophically and hatched in about 10 days (Farke, 1979). The newly hatched juveniles were ca 0.25 mm in length with a flattened, oval body shape, and had no pigment, chaetae, cirri or palps. Immediately after hatching, the juveniles dug into the sediment. Where the sediment depth was not sufficient for digging, the juveniles swam or crawled in search of a suitable substratum (Farke, 1979). In the microsystem, juvenile mortality was high (ca 10% per month) and most animals survived for less than a year (Farke, 1979). In the Wadden Sea, the majority of the cohort reached maturity and spawned at the end of their first year, although some slower developers did not spawn until the end of their second year (Farke, 1979). However, the population of Aphelochaeta marioni in Stonehouse Pool spawned for the first time at the end of the second year of life (Gibbs, 1971). There was no evidence of a major post-spawning mortality and it was suggested that individuals may survive to spawn over several years. Gibbs (1971) found that the number of eggs laid varied from 24-539 (mean=197) and was correlated with the female's number of genital segments, and hence, female size and age.

Dispersal
Under stable conditions, adult and juvenile Aphelochaeta marioni disperse by burrowing (Farke, 1979). In the microsystem, a glass barrier in the sediment prevented the movement of animals to new areas over a period of some months, even though dispersal could have occurred by creeping on the surface or swimming. When the barrier was removed, the new areas were soon colonized (Farke, 1979). Farke (1979) reported that Aphelochaeta marioni (studied as Tharyx marioni) was capable of swimming but only did so under abnormal circumstances, e.g. when removed from the sediment. Farke (1979) suggested that as there was no pelagic stage, dispersal and immigration to new areas must mainly occur during periods of erosion when animals are carried away from their habitat by water currents.